A multi-layer printed circuit board having a conductive pad disposed on the surface of the circuit board for mounting an electric part is, for example, disclosed in Japanese Patent Application Publication No. 2000-349447. In the multi-layer printed circuit board, a crack may be generated at a periphery of the conductive pad by a difference of thermal expansion. A proceeding of the crack is prevented by a conductive pattern embedded in an insulation resin and disposed near the periphery of the conductive pad.
In some cases, an electric part is mounted not only on the surface of the multi-layer printed circuit board but also inside of the multi-layer printed circuit board. One of the electric parts mounted inside of the circuit board is a thin film resistor formed by a sheet forming method, a paste forming method, a sputtering method or the like.
FIG. 9 shows a comparison sample of a multi-layer printed circuit board having a thin film resistor embedded in the circuit board, and is a schematic cross sectional view showing a multi-layer printed circuit board 100 having a thin film resistor 4 embedded therein.
The multi-layer printed circuit board 100 shown in FIG. 9 includes a resin substrate 1. The resin substrate 1 is formed by bonding five resin films together. The five resin films are made of thermoplastic resin, and are laminated. A conductive pattern 2 made of metallic film is formed on the resin film. One of the resin films laminated and bonded together includes the thin film resistor 4 and an electrode 5 for the thin film resistor 4. The one resin film is laminated and bonded so that the thin film resistor 4 is embedded in the resin substrate 1. Here, reference No. 3 represents a conductive material member filled in a hole disposed in the resin substrate 1. The conductive material member 3 connects the electrode 5 of the thin film resistor 4 and the conductive pattern 2.
The film thickness of the thin film resistor 4 is equal to or thinner than 10 μm. Specifically, in a case where the thin film resistor 4 is formed of a sheet of nickel (i.e., Ni) and phosphorous (i.e., P), the film thickness of the thin film resistor 4 is typically equal to or thinner than 1 μm. Therefore, the thin film resistor is extremely thin. Further, since the thin film resistor 4 is formed by a paste forming method or a sputtering method, strength of the thin film resistor 4 becomes small compared with the conductive pattern 2 made of metallic film. Therefore, when the resin films are laminated to manufacture the multi-layer printed circuit board 100, a crack 9 is easily generated in the thin film resistor 4 near the periphery of the electrode 5, as shown in FIG. 9.